In recent years, attention has been given to hybrid vehicles and electric vehicles against the background of environmental issues. These vehicles are equipped with an electric motor as a drive power source, and are also equipped with, e.g., a secondary battery as its electric power source. Since overdischarge and overcharge of the secondary battery lower performance of the secondary battery, it is necessary to control appropriately the charge/discharge of the secondary battery.
Japanese Patent Laying-Open No. 2006-149181 has disclosed a current control device that can prevent sudden voltage lowering in a battery. This current control device includes current sensing means for sensing a current discharged from the battery, and control means for controlling the current discharged from the battery based on a current value obtained by the current sensing means. The control means squares the current value obtained by the sensing means, and further integrates the square value on a time series to obtain a current square integrated value. Further, the control means limits the currents discharged from the battery, based on the current square integrated value. By limiting the current discharged from the battery based on the current square integrated value, it is possible to limit the discharge current value before the battery voltage lowers suddenly. In this manner, the sudden voltage lowering can be prevented when a large current is discharged.
In the current control device disclosed in Japanese Patent Laying-Open No. 2006-149181, when a failure occurs in the current sensing means, a correct current value cannot be obtained. When the charge/discharge control of the battery is performed based on an incorrect current value, it may be impossible to take out a desired power from the battery, or the battery may be overdischarged. However, Japanese Patent Laying-Open No. 2006-149181 has not disclosed a possibility of occurrence of these problems.